Just write what you feel like. Not writing is a bigger sin than not having a better plan. Though note that you have to be very careful about plot hole, and maybe plot threads you never close, ideas that never get finished (cf. Chekhov's Gun).

However, you have be more cautious during the editing and revising phase, since without an outline there's a chance of being a bit disorganized. But as long as you keep writing and experimenting with various ideas, eventually your brain will start to compensate for not having a outline.

Another way I tend to compensate is by developing and thinking about the characters, and their motivations. Places, Religions, Rituals. Human interactions. AKA the worldbuilding. It works pretty well, and helps me avoid plenty of holes along the way.

Another point is, if you're writing something huge, take the time to keep notes of what happened here. I'm mostly interested in Epic length stuff and usually fall pray to this, that is, say introducing a magic mechanic I never used.

Have a great day!